The following terminology will be used in the specification.    (*) “PDEV” is the abbreviation for a physical storage device (typically a nonvolatile storage device). The PDEV, for example, comprises a storage medium (typically a nonvolatile storage medium) and a controller for controlling an input/output (I/O) of data thereto. A nonvolatile storage medium may be a hard disk group (one or more hard disks) or an NVM group (one or more nonvolatile semiconductor memories (NVM)). That is, the PDEV, for example, may be a hard disk drive (HDD) or an NVM device such as a flash memory (FM) device (for example, a solid state device (SSD)). The NVM, for example, may be an FM or a phase-change memory. The FM, for example, may comprise multiple blocks (physical blocks), and each block may comprise multiple pages (physical pages). The NVM (typically an FM) may be a type of NVM (for example, a NAND-type FM) in which data is written in page units, data cannot be written to the page until the data in the page has been erased, and the data is erased in block units.    (*) “VOL” is the abbreviation for a logical volume, and is a logical storage device. The VOL may be a real VOL (RVOL) or a virtual VOL (VVOL). Furthermore, the VOL may be an online VOL, which is provided to an external apparatus (for example, a host computer) coupled to a DKC comprising this VOL, or an offline VOL, which is not provided to an external apparatus (is not recognized by the external apparatus). The “RVOL” is a VOL, which is based on a physical storage resource (for example, a RAID (a Redundant Array of Independent (or Inexpensive) Disks) group, which comprises multiple PDEV (nonvolatile physical storage devices)) of the storage apparatus that comprises this RVOL. As the “VVOL”, for example, there may be an externally coupled VOL (EVOL), which is a storage virtualization technology-compliant VOL that is based on a storage resource (for example, a logical volume) of an external storage apparatus coupled to the storage apparatus comprising this VVOL, or a capacity virtualization technology (typically Thin Provisioning)-compliant VOL (TPVOL), which comprises multiple virtual pages (virtual storage areas). The TPVOL is typically an online VOL. A real page is allocated to the TPVOL from a pool. The “pool” is a storage resource, which comprises multiple real pages (substantial storage areas). The pool, for example, comprises multiple pool VOLs, and each pool VOL is partitioned into two or more real pages. The “pool VOL” is a VOL, which is a component of the pool. The pool VOL may be a RVOL or an EVOL. The pool VOL is typically an offline VOL.    (*) “PVOL” is the abbreviation for a primary logical volume, and is a term denoting a copy-source VOL. “SVOL” is the abbreviation for a secondary logical volume, and is a term denoting a copy-destination VOL. A pair comprising a PVOL and an SVOL is typically a VOL pair. Data is copied from the PVOL to the SVOL.    (*) “DKC” is a term denoting a storage apparatus. The DKC comprises a VOL, and a controller for controlling a data I/O with respect to the VOL. The DKC may comprise either one or multiple PDEVs (for example, either one or multiple RAID groups), and the VOL of the DKC may be a RVOL or a VVOL.    (*) “PDKC” is the abbreviation for a primary DKC, and is a DKC that comprises a PVOL. “SDKC” is the abbreviation for a secondary DKC, and is the abbreviation of a DKC that comprises an SVOL. A remote computer system, typically, comprises a PDKC and a SDKC, and a PVOL and an SVOL that comprise a VOL pair exist in different DKCs.    (*) “MS” is the abbreviation for migration source. For example, “MS-PDKC” signifies the migration-source PDKC.    (*) “MD” is the abbreviation for migration destination. For example, “MD-PDKC” signifies the migration-destination PDKC.
A remote copy system that has a PDKC and an SDKC, and copies from the PVOL of the PDKC to the SVOL of the SDKC is known. A remote copy may include a synchronous remote copy for synchronizing the writing of data to the PVOL with the writing of this data to the SVOL, and an asynchronous remote copy for not synchronizing the writing of data to the PVOL with the writing of this data to the SVOL.
For example, Patent Literature 1 discloses a technology related to the asynchronous remote copy, that is, a technology for migrating a PVOL to a difference DKC.